Neutronics/Thermal-hydraulics Coupling in LWR Technology
(CRISSUE-S)

Background

Controlled fission power has been utilised for electricity production
worldwide in nuclear power plants (NPPs) based on light water reactor
(LWR) technology for several decades. It has proven its efficiency and
safety during these years and has manifested itself to be a reliable
and durable energy source. The foundation pillar in the peaceful
utilisation of fission nuclear power has always been the strong
emphasis on safety. Safety has been accomplished by continuously
pursuing in-depth reviews and re-evaluation of safety-related issues
incorporating findings from ongoing nuclear safety research activities
worldwide. Specific requirements have been deployed at the design and
in the permissible operation conditions of the NPPs in order to always
ensure adequate margins against critical system conditions, thus
preventing the occurrence of accidents. It is realised that as new
findings and analysis capabilities become available safety will be
increased, and it is further possible that the safety margins presently
employed will eventually be relieved (decreased) without compromising
the actual safety. Prevention and mitigation measures, however, must be
properly balanced with cost-reduction needs.

A thorough knowledge of fundamental issues – in the present case the
interaction between neutronics and thermal-hydraulics – allows pursuing
the goal of ensuring safety at reasonable costs.

Consistent with this goal, the CRISSUE-S project was created with the
aim of re-evaluating fundamental technical issues in the technology of
LWRs. Specifically, the project seeks to address the interactions
between neutron kinetics and thermal-hydraulics that affect neutron
moderation and influence the accident performance of the NPPs. This is
undertaken in the light of the advanced computational tools that are
readily available to the scientific community today.

Specifically, the CRISSUE-S activity deals with the control of fission
power and the use of high burn up fuel; these topics are part of the EC
Work Programme as well as that of other international organisations
such as the OECD/NEA and the IAEA. The problems of evaluating
reactivity induced accident (RIA) consequences and eventually deciding
the possibility of NPP prolongation must be addressed and resolved. RIA
constitutes one of the most important of the “less-resolved” safety
issues, and treating this problem may have huge positive financial,
social and environmental impacts. Public acceptance of nuclear
technology implies that problems such as these be satisfactorily
resolved.

Cross-disciplinary (regulators, industry, utilities and research
bodies) interaction and co operation within CRISSUE-S provides results
which can directly and immediately be beneficial to EU industry.
Co-operation at an international level: the participation of the EU,
former Eastern European countries, the USA, and observers from Japan
testify to the broad interest these problems engender. Competencies in
broad areas such as thermal-hydraulics, neutronics and fuel, overall
system design and reactor surveillance are needed to address the
problems that are posed here. Excellent expertise is available in
specific areas, while limited knowledge exists in the interface zones
of those areas, e.g. in the coupling between thermal-hydraulics and
neutronics. In general terms, the activities carried out and described
here aim at exploiting available expertise and findings and gathering
together expert scientists from various areas relevant to the issues
addressed.

Added value for the CRISSUE-S activity consists of proposing and making
available a list of transients to be analysed by coupled neutron
kinetics/thermal-hydraulic techniques and of defining “acceptability”
(or required precision) thresholds for the results of the analyses. The
list of transients is specific to the different NPP types such as PWR,
BWR and VVER. The acceptability thresholds for calculation precision
are general in nature and are applicable to all LWRs. The creation of a
database including the main results from coupled 3-D neutron
kinetics/thermal-hydraulic calculations and their analysis should also
be noted.
The CRISSUE-S project is organised into three work packages (WPs). The
first WP includes activities related to obtaining and documenting
relevant data. The second WP is responsible for the state-of-the-art
report (SOAR), while the third WP concerns the evaluation of the
findings from the SOAR and includes outcomes of the entire project
formulated as recommendations, mainly to the nuclear power industry and
to the regulatory authorities. The present report is the result of the
first WP and discusses the type of transients that are of interest in
relation to reactivity-initiated accidents in LWR NPPs and elaborates
on the data required for coupled 3-D neutron kinetics/thermal hydraulic
analysis and also on data needed to perform associated validations.

Comprehensive reports composed of
contributions from the different CRISSUE S participating organisations,
unavoidably imply non-homogeneous treatment of the various topics,
although an effort was made to provide a consistency between the
various sections. However, it is realised that the adopted level of
detail is not commensurate with the safety relevance or the
technological importance of the issues discussed.

The reports have been written to accomplish the objectives established
in the contract between the EU and its partners. Expected beneficiaries
include institutions and organisations involved with nuclear technology
(e.g. utilities, regulators, research, fuel industry). In addition,
specific expected beneficiaries are junior- or senior-level researchers
and technologists working in the considered field of research and
development and application of coupled neutron
kinetics/thermal-hydraulics.

Six plenary CRISSUE-S meetings took place over the course of the
project implementation period. The meetings were held at:

The importance of the CRISSUE-S project has been expressed by the
OECD/NEA Nuclear Science Committee. This interest has also been
emphasised by the OECD/NEA Committee on Nuclear Safety, as the project
discusses many of their activities. It was agreed that the CRISSUE-S
reports be published by the OECD/NEA as their contribution to the
project.

The reports were produced by the members of the
CRISSUE-S project for use within their organisations. The present
versions are made widely available for the greater benefit of
organisations and experts working in the nuclear power area.